The primary objective of this collaborative double-blind placebo- controlled trial is to determine, using a factorial design, whether aspirin (324 mg/day) or lovastatin (20-40 mg/day), over an average treatment period of three years, retards the progression of atherosclerotic plaques in the carotid arteries of high risk individuals with asymptomatic carotid stenosis. Men and women free of clinically evident coronary and cerebrovascualr disease who are 50 years of age or older, with total serum cholesterol levels between approximately the 50th (220 mg/dl) and 90th (260 mg/dl) percentiles and at least one plaque with a thickness between 1.0 and 4.0 mm are eligible for participation. A secondary objective is to determine the safety of long-term therapy with the two active interventions, individually and in combination. Coronary and cerebrovascular events will be monitored and natural history studies conducted. Progression of the atherosclerotic lesions will be determined semi-annually. Mean wall thickness from the far wall of the carotid artery (common, bifurcation and internal) will be the primary outcome measure. Maximum wall thickness/minimum lumen diameter will also be determined as well as a global extent score. The goal of the lipid-lowering treatment is to bring total serum cholesterol below 180 mg/dl. A favorable outcome of the trial will have major public health implications for the prevention/control of atherosclerosis and its complications. The successful employment of B-mode imaging will greatly impact on health care (ability to monitor plaque progression non-invasively in individuals) and permit a reduction in cost and time for future clinical trials evaluating treatment for atherosclerosis.